Silicon wafers are the building blocks of a wide variety of semiconductor devices, such as solar cells, integrated circuits, and MEMS devices. For example, Evergreen Solar, Inc. of Marlboro, Mass. forms solar cells from silicon wafers fabricated by means of the well known “ribbon pulling” technique.
The ribbon pulling technique generally uses a specialized furnace that surrounds a crucible containing molten silicon and a growing ribbon crystal. This furnace generally has a base formed from a solid, insulating material, and insulation (known as an “afterheater”) positioned both above the base insulation and near a growing ribbon crystal.
In general, prior art afterheaters known to the inventors provide only limited variability of the temperature profile in its immediate vicinity. Consequently, as it cools, a proximate growing ribbon crystal may contain stresses that detrimentally affect its physical properties. This stress undesirably can reduce the conversion efficiency of a solar cell ultimately made from such a ribbon crystal.